Thermal barrier coating method and article

ABSTRACT

An outwardly grown diffusion aluminide bondcoat is formed on a superalloy substrate and has higher concentrations of Al and Pt and lower concentrations of harmful impurities (e.g. Mo, W, Cr, Ta, S, etc.) at an outermost region of the bondcoat than at an innermost region thereof adjacent the substrate. The bondcoat is pretreated prior to deposition of a ceramic thermal insulative layer in a manner that reduces grain boundary ridges on the outermost bondcoat surface without adversely affecting the outermost region thereof, and then is heat treated to thermally grow a stable alpha alumina layer on the bondcoat prior to deposition of a ceramic layer.

This is a division of Ser. No. 09/511,857 filed Feb. 23, 2000 now U.S.Pat. No. 6,472,018.

FIELD OF THE INVENTION

The present invention relates to thermal barrier coatings forcomponents, such as gas turbine engine blades and vanes, wherein thethermal barrier coating exhibits improved coating life.

BACKGROUND OF THE INVENTION

Advancements in propulsion technologies have required gas turbineengines to operate at higher temperatures. This increase in operatingtemperature has required concomitant advancements in the operatingtemperatures of metal (e.g. superalloy) turbine engine components.Thermal barrier coatings have been used to meet these higher temperaturerequirements. Typical thermal barrier coatings comprise alumina and/orzirconia based ceramic which provide a thermal insulative layer toprotect the metal component from the high temperatures.

Thermal barrier coatings have been applied to metal components by firstcoating the component with a bondcoat, which may comprise an inwardly oroutwardly grown platinum modified diffusion aluminide bondcoat and/orMCrAlY overlay bondcoat where M is Ni and/or Co. After applying thebondcoat, the coated component typically is grit blasted and vacuum heattreated, or vice versa, to promote the formation of a thermally grownoxide (TGO) layer typically comprising alumina on the aluminum-richunderlying bondcoat. The component then is coated by electron beamphysical vapor deposition with a thermal insulative layer of alumina,zirconia, or other ceramic material. For example, U.S. Pat. Nos.5,716,720 and 5,856,027 describe a thermal barrier coating systemcomprising a clean platinum modified diffusion aluminide bondcoat on thesubstrate, a thermal grown alumina layer, and a thermal insulativeceramic layer on the alumina layer. The platinum modified diffusionaluminide bondcoat comprises an outwardly grown diffusion aluminidecoating produced by CVD (chemical vapor deposition) processing usinghigh substrate temperature and low activity coating gases that producehigher concentrations of Pt and Al and low concentrations of harmfulrefractory metal impurities (e.g. Mo, W, Cr, Ta, etc.) and surfaceactive impurities (e.g. S, P, Cl, B, etc.) at the outermost zone orregion of the aluminide coating.

The life of a thermal barrier coating; i.e. time to coating spallation,is known to be related to the surface characteristics of the bondcoatand the particular phase of the thermally grown alumina present betweenthe insulative layer and the bondcoat. Negative effects of bondcoatsurface roughness on coating life have been reported by Jordan in“Bondcoat Strength and Stress Measurements in Thermal Barrier Coatings”,US Department of Energy Report (subcontract # 95-01-SR030), Sep. 30,1997, where it was reported that platinum aluminide bondcoat surfacesinclude grain boundary ridges that act as sites for stress concentrationand damage accumulation during thermal cycling of thermal barrier coatedsubstrates. Grain boundary ridges act as sites for preferentialoxidation, void formation, and crack initiation which result in thethermally grown alumina spalling prematurely.

The formation of the thermally grown alumina layer on the aluminum-richbondcoat involves several metastable transition phases, such as a cubicgamma alumina phase transforming to a tetragonal delta alumina phasethen to a monoclinic theta alumina phase finally to a rhombohedral alphaalumina phase, the formation of which occurs by heterogeneous nucleationand growth of the alpha phase from the monoclinic theta phase. The sumof the metastable transitions involves a substantial molar volumereduction of approximately 9%, a significant portion of which isattributable to the final transition from the theta phase to alphaphase.

An object of the present invention is to provide a method of pretreatinga superalloy or other substrate prior to coating with a thermalinsulative layer of a thermal barrier coating system in a manner toreduce adverse effects of bondcoat surface roughness on life of thethermal barrier coating.

Another object of the present invention is to provide a method ofpretreating a superalloy or other substrate prior to coating with athermal insulative layer of a thermal barrier coating system in a mannerto reduce adverse effects of metastable phases of thermally grownalumina on life of the thermal barrier coating.

A still further object of the present invention is to significantlyincrease the life of the thermal barrier coating system under hightemperature cyclic oxidation conditions.

SUMMARY OF THE INVENTION

The present invention involves forming on a superalloy or other metallicsubstrate an outwardly grown diffusion aluminide bondcoat that hashigher concentrations of Al and optionally Pt and lower concentration ofa harmful refractory metal impurity (e.g. Mo, W, Cr, Ta, etc.) at anoutermost zone or region of the bondcoat than at an innermost zone orregion thereof and pretreating the bondcoat by repetitively moving thebondcoated substrate in contact with abrasive media in a container toreduce bondcoat grain boundary ridges, while leaving at least a portion,preferably all, of the outermost Al-rich zone or region of the originalAl-rich bondcoat. The repetitive moving can be achieved by media bowlpolishing.

The present invention also involves heat treating the pretreatedbondcoated substrate under temperature and time conditions in air toform a stable alpha alumina layer on the bondcoat prior to deposition ofthe ceramic thermal insulative layer.

A thermal barrier coating system formed pursuant the present inventionexhibits a significant increase in coating life (time to coatingspallation) in cyclic oxidation tests at elevated temperatures.

The above objects and advantages of the present invention will becomemore readily apparent from the following description taken with thefollowing drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of concentrations, in weight %, of Pt, Al, Cr, W, Ta,and Mo across the thickness of a platinum modified diffusion aluminidebondcoat with 0 thickness being the outer surface of the bondcoat.

FIG. 2 is a bar graph of average cycles to failure of thermal barriercoating systems wherein the bondcoat was pretreated by vapor honing,grit blasting and media bowl polishing pursuant to the invention.

FIG. 3 is a graph of average cycles to failure of thermal barriercoating systems wherein the bondcoat was pretreated by media bowlpolishing for different times.

FIG. 4A is a photomicrograph of a Pt modified diffusion aluminidecoating before media polishing, FIG. 4B is a photomicrograph of the Ptmodified diffusion aluminide coating after media polishing for 10minutes, and FIG. 4C is a photomicrograph of the Pt modified diffusionaluminide coating after media polishing for 30 minutes.

FIG. 5A is a photomicrograph of a Pt modified diffusion aluminidecoating after vapor honing at 20 psi, and FIG. 5B is a photomicrographof the Pt modified diffusion aluminide coating after vapor honing at 60psi.

FIG. 6A is a photomicrograph of a Pt modified diffusion aluminidecoating after grit blasting at 20 psi, and FIG. 6B is a photomicrographof the Pt modified diffusion aluminide coating after grit blasting at 40psi.

FIG. 7 is a time-temperature-transition diagram for alpha aluminaformation on an untreated (unworked) outwardly grown platinum modifieddiffusion aluminide.

FIG. 8 is a graph of air oxidation time (transformation finish time)needed to form stable alpha alumina in air at 1975 degrees F. versus thetime of media bowl polishing.

DESCRIPTION OF THE INVENTION

An exemplary embodiment of the present invention involves forming on asuperalloy or other metallic substrate an outwardly grown platinummodified diffusion aluminide bondcoat that has higher concentrations ofAl and Pt and lower concentrations of harmful refractory metalimpurities (e.g. Mo, W, Cr, Ta, etc.) at an outermost zone or region ofthe bondcoat than at an innermost zone or region thereof and pretreatingthe bondcoated substrate in a manner that reduces bondcoat grainboundary ridges, and then to form a stable alpha alumina layer on thebondcoat prior to deposition of the thermal insulative layer in a mannerthat significantly increases coating life.

The substrate can comprise nickel and cobalt superalloy and othermetallic substrates which may comprise equiaxed, directionallysolidified and single crystal castings as well as other forms of thesematerials, such as forgings, pressed powder components, machinedcomponents, and other forms. For example only, the substrate maycomprise the well known Rene' N5 nickel base superalloy having acomposition of Ni-7.0% Cr-6.2% Al-7.5% Co-6.5% Ta-1.5% Mo-5.0% W-3.0%Re-0.15% Hf-0.05% C-0.018% Y (where % is in weight %) used for makingsingle crystal turbine blades and vanes. Other nickel base superalloyswhich can be used include, but are not limited to, MarM247, CMSX-4, PWA1422, PWA 1480, PWA 1484, Rene' 80, Rene' 142, and SC 180. Cobalt basedsuperalloys which can be used include, but are not limited to, FSX-414,X-40, MarM509 and others.

The bondcoat preferably comprises an outwardly grown platinum modifieddiffusion aluminide bondcoat that has higher concentrations of Al and Ptand lower concentrations of harmful refractory metal impurities (e.g.Mo, W, Cr, Ta, etc.) at the outermost zone or region of the bondcoatthan at the innermost zone or region thereof. The outwardly grownplatinum modified diffusion aluminide bondcoat is formed by firstelectroplating a platinum layer on the substrate using an alkali oralkaline earth hydroxide platinum plating solution as described in U.S.Pat. No. 5,788,823, the teachings of which are incorporated herein byreference. The platinum coated substrate then is subjected to a chemicalvapor deposition (CVD) process described in U.S. Pat. Nos. 5,658,614 and5,989,733, the teachings of which are incorporated herein by reference.The CVD process described in the these patents uses an aluminumtrichloride coating gas in a hydrogen carrier gas at a substrate coatingtemperature of at least 1832 degrees F. to form the outwardly grownplatinum modified diffusion aluminide bondcoat. The bondcoat comprisesan innermost diffusion zone or region adjacent the substrate and anoutermost additive layer zone or region formed by outward diffusion ofnickel from the substrate and its subsequent reaction with aluminum fromthe coating gas (AlCl₃)

An exemplary outwardly grown platinum modified diffusion aluminidebondcoat pursuant to the invention (designated MDC-150L) formed on Rene'N5 nickel base superalloy has higher concentrations of Pt and Al andlower concentrations of harmful refractory metal impurities (e.g. Mo, W,Cr, Ta, etc.) at the outermost region (additive layer) of the bondcoatthan at the innermost region thereof as illustrated in FIG. 1, which isa graph of concentrations, in weight %, of Pt, Al, Cr, W, Ta, and Moacross the thickness of the platinum modified diffusion bondcoat whereabout 0 microns to 30 microns distance inwardly through the bondcoatcorresponds to the outermost additive layer or zone and about 35 micronsto about 50 microns inwardly corresponds to an innermost diffusion zone.The outwardly grown platinum modified diffusion aluminide coatings ofthe type described above and in U.S. Pat. Nos. 5,658,614 and 5,989,733are preferred since they include the outermost additive layer or zonethat is relatively clean and purified (i.e. having reducedconcentrations of refractory metal and surface active impurities) as aresult of outward coating growth and purification by gas-solid reactionsduring CVD coating such that a pure, thermodynamically stable alphaalumina scale or layer can be formed thereon by heat treatment in airwithout transient oxidation or spinel formation. Since refractory metalimpurities in the bondcoat hinder formation of the desired high purityalpha alumina phase on the bondcoat, their reduced concentration in theoutermost additive layer helps minimize harmful doping effects thatrefractory elements may produce when dissolved in the alpha aluminascale. In addition, reduction of surface active impurities improvesaluminum adherence to the bondcoat.

In the outer additive layer of the bondcoat, the Pt and Alconcentrations typically are in the ranges of 12 to 30 weight % Pt and15 and 28 weight % Al, respectively, while being 0 to 8 weight % Pt and4 to 12 weight % Al, respectively, at the innermost region of thebondcoat adjacent the substrate. The refractory metal impurities, suchas Mo, W, Cr, Ta, etc.) typically each are present in an amount lessthan about 1 weight % for Mo, W, and Ta and less than 3 weight % for Crin the outermost additive layer, while being present at their respectivenominal alloy concentrations in the diffusion zone at the innermostdiffusion zone or region adjacent the substrate. The refractory metalimpurities in the bondcoat originate from the underlying superalloysubstrate via outward diffusion from the substrate.

For purposes of illustration and not limitation, the exemplary bondcoatchemistry profile shown in FIG. 1 was formed on the Pt plated Rene' N5substrate using the following coating parameters: coating gas comprising9 volume % aluminum trichloride and 91 volume % hydrogen at a flow rateof 300 scfh and total pressure of 150 torr and substrate temperature of1975 degrees F. for a coating time of 16 hours without prediffusion of aplatinum layer thereon. Prior to CVD aluminizing, the substrate waselectroplated with the platinum layer comprising 9-11 milligrams/cm² ofPt using an alkali or alkaline earth hydroxide plating bath having abath composition comprising an aqueous KOH solution with 10 grams Pt perliter and electrical current of less than 20 mA/cm², as taught in U.S.Pat. No. 5,788,823 incorporated herein by reference.

In accordance with an embodiment of the invention, the outwardly grownplatinum modified diffusion aluminide bondcoat on the substrate istreated in a manner that reduces grain boundary ridges on the outermostbondcoat surface, while leaving at least a portion and preferably all ofthe Pt and Al-rich outermost zone or region (outer additive layer), andthen to form a stable alpha alumina layer on the bondcoat prior todeposition of the ceramic thermal insulative layer thereon.

For purposes of illustration and not limitation, the outermost surfaceof the bondcoat was subjected to different surface treatments comprisingmedia bowl polishing and vapor honing. The outermost surface of thebondcoat also was grit blasted for comparison purposes representative ofconventional grit blasting treatment used heretofore to eliminatetransient oxide formation on both inwardly and outwardly grown Ptmodified diffusion bondcoats and MCrAlY type overlay bondcoatspreparatory to deposition of ceramic insulative coating.

Media bowl polishing of the bondcoat surface pursuant to an embodimentof the invention involved placing the bondcoated substrate in a bowlhaving commercially available angle cut, cylindrical alumina polishingmedia of ⅝×⅝ inch particle size and vibrating the bowl such that thebondcoat outermost surface was vibratory polished for a time of 5minutes. Media bowl polishing of the bondcoat was conducted in equipmentand using alumina media commercially available from Sweco Inc.,Florence, Ky. For example, a Sweco model FMD20HA media bowl polishingdevice operates at a frequency of vibration of 1200 cycles/minute with avertical amplitude range of ⅛-⅜ inch and horizontal amplitude range of⅛-¼ inch. Media bowl polishing repetitively moves the bondcoatedsubstrate and abrasive media in contact in a container (bowl) byvibration of the container.

Vapor honing of the bondcoat surface involved impinging the bondcoatoutermost surface with vapor comprising water and commercially available−600 grit Novaculite particles at a pressure of 30 psi for a time of 5minutes. Vapor honing of the bondcoat was conducted in equipmentcommercially available from Vapor Blast Manufacturing Company,Milwaukee, Wis.

Grit blasting of the bondcoat surface involved impinging the bondcoatoutermost surface with abrasive alumina grit particulates having aparticle size of 220-240 grit at a pressure of 20 psi for a time of ¾minute. Grit blasting of the bondcoat was conducted in equipmentcommercially available from Empire Abrasive Equipment Company, Langhorn,Pa.

After media bowl polishing, the bondcoated substrates were subjected toheat treatment at 1975 degrees F. for sufficient time (determined fromFIG. 8) in air to form a stable thermally grown alpha alumina layer andthen conventionally coated in an electron beam-physical vapor deposition(EB-PVD) coater with yttria stabilized zirconia insulative layer to athickness of 0.005-0.007 inch (127-178 micrometers). Grit blastedsamples received a vacuum heat treatment at 1925 degrees F. for 2 hourssimilar to U.S. Pat. No. 5,716,720 prior to coating. Vapor honed sampleswere treated with molten KOH to remove embedded Novaculite particles andair heat treated at 1975 degrees F. for 3.75 hours prior to coating.

The heating time in air that is sufficient to form stable alpha aluminaon the coating of media bowl polished samples depended on the time ofmedia bowl polishing (i.e. amount of work imparted to the coatingsurface), FIG. 8, where it can be seen that increased media bowlpolishing time decreases the time needed to form the stable alphaalumina at 1975 degrees F.

The thermal barrier coated substrates then were tested in cyclicoxidation in test cycles where each cycle was 60 minutes in durationconsisting of exposure to 2075 degrees F. for 50 minutes in air followedby 10 minutes of cooling in air to below 400 degrees F. A thermalbarrier coated substrate was considered failed when 20% of the thermalbarrier coating on the outermost surface was spalled. The cyclicoxidation tests involved testing 3 specimens of each thermal barriercoated substrate with the lives of the specimens averaged and appearingin FIGS. 2 and 3.

FIG. 2 summarizes the cyclic oxidation test results where it can be seenthat the media bowl polishing pursuant to the invention produced thelongest coating life of the three bondcoat surface treatments conducted.The significant advantage of the media bowl polishing over the vaporhoned and grit blasted bondcoated substrates results from reduction orelimination of raised grain boundary surface ridges on the outermostsurface of the bondcoat without removing any of the Pt and Al-richoutermost additive layer or region of the bondcoat as shown in FIGS. 4Band 4C compared to FIG. 4A. That is, the grain boundary ridges arepreferentially removed in FIG. 4B as evidenced by abrasive scratch markson the raised ridges and not on the grains enclosed by the grainboundary ridges. In FIG. 4C, abrasive scratch marks are present on thegrain boundary ridges and some of the grain surfaces, indicating thatthe ridges are nearly removed level with the grain surfaces at the outersurface of the bondcoat without removing any of the Al and Pt-rich outeradditive layer of the bondcoat. Media bowl polishing for 5 minutesimproved the life (e.g. 742 cycles) of the thermal barrier coatedsubstrate by 42% in the cyclic oxidation tests as compared to that (e.g.525 cycles) of similar thermal barrier coated substrate prepared withoutmedia bowl polishing; i.e. with no surface treatment except airpreoxidation.

In contrast, vapor honing and grit blasting removed substantial portionsof the Pt and Al-rich outermost additive layer or region as shown inFIGS. 5A, 5B and 6A, 6B in a manner that adversely affected coating lifein the cyclic oxidation tests. Both the vapor honing and grit blastingsurface treatments failed to produce an improvement in the life of thethermal barrier coated substrate as compared to that of a similarthermal barrier coated substrate prepared with no surface treatment atall. FIGS. 5 and 6 evidence gross removal [e.g. tenths of a mil(mil=0.001 inch) of bondcoat removed] of the bondcoating itself to thedetriment of oxidation resistance.

FIG. 3 illustrates life of the thermal barrier coated substratesprepared as above using media bowl polishing pursuant to the inventionwith, however, the time of media bowl polishing being varied. In thesecyclic oxidation tests, the media bowl polished bondcoated substrateswere heat treated at 1975 degrees F. for sufficient time in air, FIG. 8,to form a completely transformed alpha alumina layer or scale on thebondcoat before deposition of the yttria stabilized zirconia layer.Although the life of the thermal barrier coated substrates started todecrease with increasing media bowl polishing beyond about 5 minutes,media bowl polishing for about 3 to about 10 minutes significantlyimproved the life of the thermal barrier coated substrate. Even after 30minutes of media bowl polishing, the thermal barrier coated substratesstill exhibited better life (e.g. about 575 cycles) in the cyclicoxidations tests as compared to that (e.g. 525 cycles) of similarthermal barrier coated substrate prepared without media bowl polishing;i.e. with no surface treatment except air preoxidation.

Although the invention has been described hereabove with respect toforming an outwardly grown Pt modified diffusion aluminide coating byCVD processing, the invention is not so limited and can be practiced byforming an outwardly grown simple diffusion aluminide coating devoid ofPt on the substrate using CVD processes of the type described above.Also, the outwardly grown diffusion aluminide coatings may include oneor more active elements selected from Hf, Zr, Si, Y, La, and Ce with orwithout platinum and also can be made by CVD, pack, above-the-pack andother vapor phase coating processes.

The present invention also involves heat treating the pretreatedbondcoated substrate under temperature and time conditions in anoxygen-bearing atmosphere to form a thermodynamically stable alphaalumina layer on the bondcoat prior to application of the thermalinsulative layer. Rene' N5 substrates coated with the above bondcoatwere subjected to various air heat treatments at 1950, 1975, 2000, and2150 degrees F. for various times. The resulting transformation startT_(s) and transformation finish T_(f) times with respective temperaturesfor the formation of alpha alumina on the bondcoat are shown in Table I.

TABLE I Temperature Transformation Start Transformation Finish (degreesF.) (T_(s)) (T_(f)) 1950 1 hour about 8 hours 1975 30 minutes less than1 hour 2000 30 minutes less than 1 hour 2150 less than less than 10minutes 30 minutes

Table I reveals that a stable alpha alumina layer can be formed on thebondcoat by heat treatment at temperatures of 1950 degrees F. and abovewithin approximately 8 hours and less. Higher temperatures considerablyreduced the heat treatment time to form the fully transformed alphaalumina layer on the bondcoat.

The T_(s) and T_(f) data points were chosen with respect to the time atwhich the stable alpha alumina phase peaks (T_(s)) were first detectedby X-ray diffraction analysis and when the last theta alumina peaks wereno longer detectable (T_(f)). From these air heat treatment data points,a time-temperature-transformation (TTT) diagram was developed and isshown in FIG. 7. In FIG. 7, the 2150 degree F. T_(s) was extrapolated.

Further, Rene' N5 substrates coated with the CVD Pt aluminide MDC-150Lbondcoats without any bondcoat surface treatment were preoxidized in airat 1975 degrees F. for 30 minutes and then examined by X-ray diffractionboth prior to and after a standard pre-heat cycle (i.e. 40 minutes at1950 degrees F. in vacuum of 6 microns) employed in the conventionalEB-PVD coater in which the yttria stabilized zirconia was deposited onthe bondcoat. X-ray diffraction analysis indicated that thetransformation from theta alumina to alpha alumina was not completefollowing pre-oxidation and did not proceed during the EB-PVD pre-heatoperation. These results suggest that the optimum fully transformedalpha alumina layer to maximize life of the thermal barrier coatedsubstrate cannot be formed during the EB-PVD pre-heat cycle, or anyother thermal process at reduced oxygen pressure and temperatures at orbelow 1950 degrees F. Thus, pursuant to a preferred embodiment of theinvention, the substrate is heat treated in air at temperatures of about1950 degrees F. and higher for appropriate times to form the alphaalumina layer prior to placing the bondcoated substrate in the EB-PVDcoater that is used to deposit the ceramic insulative layer. Thebondcoat can be heated in air at a temperature-in the range of about1900, preferably about 1950, to about 2200 degrees F. for 1 to 500minutes to form the alpha alumina layer. Although the invention has beendescribed in detail above with respect to certain embodiments, thoseskilled in the art will appreciate that modifications, changes and thelike can be made therein without departing from the spirit and scope ofthe invention as set forth in the appended claims.

1. A coated article, comprising a substrate and a diffusion aluminidebondcoat grown outwardly from said substrate, said bondcoat having aninnermost diffusion region adjacent the substrate, said bondcoat furtherhaving an outermost additive region comprising outwardly diffused nickelfrom the substrate reacted with aluminum, having a higher concentrationof Al and lower concentration of a refractory metal than said innermostdiffusion region, and having an outermost surface having at leastpartially removed grain boundary ridges thereon, and a thermal barriercoating disposed on said outermost surface.
 2. The article of claim 1wherein said outermost surface is a media bowl polished surface.
 3. Thearticle of claim 1 wherein said substrate is a nickel base superalloy.4. The article of claim 1 further including an alpha alumina layerformed on said outermost surface.
 5. The article of claim 4 wherein thethermal barrier coating comprises a ceramic layer disposed on thealumina layer.
 6. The article of claim 1 wherein the at least partiallyremoved grain boundary ridges on the outermost surface are nearly levelwith grain surfaces enclosed by the grain boundary ridges.
 7. A coatedarticle, comprising a substrate and a diffusion aluminide bondcoat grownoutwardly from said substrate, said bondcoat having an innermostdiffusion region adjacent the substrate, said bondcoat further having anoutermost additive region comprising outwardly diffused nickel from thesubstrate reacted with aluminum, having a higher concentration of Al andPt and lower concentration of a refractory metal than said innermostdiffusion region, and having an outermost surface having at leastpartially removed grain boundary ridges thereon, and a thermal barriercoating disposed on said outermost surface.
 8. The article of claim 7wherein the at least partially removed grain boundary ridges on theoutermost surface are nearly level with grain surfaces enclosed by thegrain boundary ridges.
 9. The coated article of claim 7 wherein saidoutermost surface is a media bowl polished surface.